Ukraine’s government troops have taken over the pro-Russian stronghold of Slovyansk, a flashpoint east Ukrainian town held by rebels since mid-April, killing five “terrorists” during operations.

A Ukrainian military detachment with five armoured personnel carriers took over a checkpoint on a road north of the city.

No shots were heard during the takeover, but the Ukrainian interior ministry said its forces had killed “up to give terrorists” while destroying the militant checkpoints.

The move has prompted Russia to order new military exercises on the Ukrainian border as Russia President Vladimir Putin warned that there would be consequences if the army was used against pro-Russian activists.

Russia, which has an estimated 40,000 troops massed on Ukraine's border, has already threatened to respond like it did when it invaded Georgia in 2008 if it sees its interests in Ukraine attacked.

“If Kiev really began to use the army against the country's population... that is a very serious crime against its own people," Putin said while warning of “consequences, including for intergovernmental relations.”

"From today, military exercises have started in regions bordering Ukraine involving battalions of tactical forces of the southern and western military districts," Russia’s defence minister said.

The operation in Slovyansk came hours after the Ukrainian government reported that its special forces took back control of the town in the southeastern city of Mariupol. Troops also repelled an attack by heavily armed rebels in the eastern town of Artemvisk.

The Defence Ministry said in a statement that Ukranian forces repelled nearly 100 separatists in an attack on the military base in Artemivsk, just north of rebel-held Donetsk.

Meanwhile, US President Barack Obama accused Russia of failing to abide by the spirit of the Geneva deal agreed last week by Ukraine, Russia, the EU and the US.

"So far we have seen them not abide by the spirit or the letter of the agreement in Geneva. If this continued, there will be further consequences and we will ramp up further sanctions, “ Obama said.

If that continued, he said, "there will be further consequences and we will ramp up further sanctions".